Liquid scintillation counting is generally known and is a widely used technique for the measurement of low energy beta emitting radionuclides in addition to gamma and alpha emitters. As is known, a sample whose emission characteristics is to be measured is mixed in a solution called a cocktail. The cocktail typically includes a solvent such as toluene, xylene, and dissolved solutes, such as fluors. The solvent absorbs the energy of the radioactive decay and transfers it to the fluors. When a fluor molecule decays to the ground state it gives off a photon of light. For every radioactive decay event there are many photons produced and the number of photons is proportional to the energy of the decay. Each light event is converted in a photomultiplier to an electrical signal so that the occurrence can be recorded in the scintillation counter.
Typically an analytical method is used in a scintillation counter to measure the radioactivity by incorporating a radiolabeled analyte into solution with the fluors in the scintillation cocktail. It is particularly important in the preparation of radioactive samples for a scintillation counter to ensure that the radiochemical sample is compatible with the scintillation medium. Emulsifier-type scintillation cocktails are designed to incorporate the aqueous sample into intimate contact with the fluors in the organic phase. If the sample is not homogeneous, local variations in radionuclide solution geometry will cause a nonreproducible sample activity. Additionally, the use of quench correction in heterogeneous samples often results in inaccurate, nonreproducible determinations of sample activity.
Several prior art methods have been proposed for measuring sample homogeneity. One such method is a dual-ratio technique described in an article published in the International Journal of Applied Radiation and Isotopes, 1968, Vol. 19, pages 447-462 by E. T. Bush for "A Double Ratio Technique As An Aid To Selection Of Sample Preparation Procedures In Liquid Scintillation Counting". This technique is based on the fact that efficiency for counting radioactive samples depends upon both the degree of cocktail quenching in the solution based on an external standard and upon the extent to which the sample material is uniformly dispersed in the cocktail. Thus, the external standard can be used to measure quenching independently of efficiency with which the radioisotope is actually being counted. A plot can be made of sample channels ratio versus external standard channels ratio for a set of standard, quenched solution samples. Unknown samples, if they are true solutions, should give values falling on this curve. Deviations from the curve in a direction of "more quench" sample ratio indicate samples are not completely dissolved. A deficiency of the dual ratio technique is that it is only applicable in single label analysis.
The impact of heterogeneous solution in creating erroneous results was also described in two articles published in Laboratory Practice, Vol. 30, No. 5, May 5, 1981 by D. Horrocks and A. Kolb in "Common Methods In Sample Preparation For Liquid Scintillation Counting" and "Instrumental Methods For Detecting Some Common Problems In Liquid Scintillation Counting". The latter article describes a liquid scintillation system which has the capability of identifying certain types of errors occurring in liquid scintillation samples, one of these which identifies a separation of the organic scintillant and aqueous sample. This particular scintillation counter involves determining the pulse height spectrum for Compton electron. If the Compton edge is discontinuous it is concluded that the sample is two-phased. Although this method is applicable to multiple radiolabeled analytes in samples, a limitation of this particular method is that two distinct liquid phases are required, both of which must scintillate. However, if only one of the two phases scintillates, i.e., adsorption on an inert support, this method will not work. Accordingly, this method would falsely identify samples of radiolabeled analytes absorbed into solid supports, such as a filter disc or sample adsorption, as homogeneous.
Any loss of physical contact between the radiolabeled analytes and the fluors of the scintillation cocktail creates a heterogeneous sample resulting in erroneous readings. Radiolabeled analytes which have become partially or totally removed from the organic phase produce different radionuclide counting efficiencies depending upon the extent of dissolution in each phase. Hydrophilic cocktails in which the aqueous holding capacity has been exceeded results in phase separation. Nearly all emulsifier-type cocktails have regions of instability in which the aqueous material is not in solution. The extent of these regions will be dependent upon temperature, volume of cocktail, the volume of the analyte, solutes in the analytes, and time. A nonpolar organic soluble radiochemical will always remain in true solution within an emulsifier cocktail although the cocktail appears to be phase separated. The geometry of the radiochemical species is essentially 4.pi. in relation to the organic scintillating phase for true solution samples. In heterogeneous samples the external standard measurement indicates physical change occurring within the organic scintillation phase. The external source of scintillation events cannot effectively reflect the geometric distribution of the radio compounds. Since the radiochemical is in close contact with the scintillation medium, changes in physical counting environment in relationship to the radionuclide distribution would be indicated by a modified sample pulse height energy distribution.